Method for fabricating ceramic composites, fibrous ceramic monoliths

ABSTRACT

A method for producing composite ceramic material is provided wherein a core ceramic structure is produced and simultaneously enveloped with a sleeve of similar material.

CONTRACTUAL ORIGIN OF THE INVENTION

[0001] The United States Government has rights in this invention underContract No. W-31-109-ENG-38 between the U.S. Department of Energy andthe University of Chicago representing Argonne National Laboratory.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for fabricatingcomposite ceramic structures, and more particularly, the presentinvention relates to a method for fabricating multi-phase ceramicfilaments to be used as building blocks for the structures.

[0004] 2. Background of the Invention

[0005] When monolithic ceramic structures are stressed to the point offailure, they often fail with little or no warning, i.e.,catastrophically. These monolithic ceramics simply load elastically to amaximum stress and then fail all at once.

[0006] Several techniques for improving the toughness of ceramicstructures have been attempted. For example, fibers have been added tobulk ceramic material to increase its toughness. These are calledcontinuous fiber ceramic composites. However, even these toughermaterials can fail catastrophically.

[0007] Attempts have been made at forming ceramic structures which fail“gracefully” (i.e., with warning). One of these structures are known asfibrous monoliths (FMs), and are fabricated from billets comprising acomposite ceramic containing both a strong cellular phase (a core)surrounded by a phase (in the form of a sleeve) designed to dissipateenergy during fracture. Current FM production processes utilizemulti-step protocols and heterogeneous materials. These processes makeuse of ram extrusion of slugs, and are thus batch processes bydefinition.

[0008] U.S. Pat. No. 4,772,524 awarded to Coblenz on Sep. 20, 1988discloses a fibrous monolith whereby a cotton thread runs co-axiallywith the monolith produced. U.S. Pat. No. 5,645,781 awarded to Popovicet al. on Jul. 8, 1997 discloses a method for clamping or wrapping asleeve of ceramic material around a dense core of ceramic material.These processes require separate steps and are thus considered “batch”operations.

[0009] Another drawback to current FM production processes is that hotpressing (often in an inert atmosphere) is generally used to densify thematerials after their formation. To date, hot pressing in an inertatmosphere has been used to densify FMs such as silicon nitride(cell)/boron nitride (cell boundary) systems and diamond-cobalt(cell)/cobalt (cell boundary) systems. These FMs have been made fromthermoplastic co-polymer blends that also require special processingduring extrusion, such as close control of pressure and need for use ofelevated temperatures above 160° C., Hot pressing limits the shapes ofthe FMs that can be produced. Also, this process is costly.

[0010] A need exists in the art for a process for producing ceramiccomposites which utilize common extrusion equipment and similar phasematerials. The process should produce a structure which yieldsgracefully. The process also should utilize common ceramic materials andsintering steps conducted in air and at ambient pressure to furtherminimize cost. Finally, the process should incorporate the least numberof steps, and preferably comprise a single step or continuous process soas to expedite production in large industrial scale scenarios.

SUMMARY OF THE INVENTION

[0011] It is an object of the present invention to provide a method forproducing ceramic composite materials that overcomes many of thedisadvantages of the prior art.

[0012] Another object of the present invention is to provide acontinuous process for producing ceramic composite material. A featureof the invented method is the use of a standard extrusion machine. Anadvantage of the invented method is that minimal fabricator training isrequired, resulting in an optimization of personnel and existingequipment, and ultimately lower costs.

[0013] Yet another object of the present invention is to provide aprocess for producing robust unidirectional or multidirectional fibrousmonoliths. A feature of the invented method is careful matching ofshrinkages and thermal-expansion coefficients of the various phasescomprising the monoliths. An advantage of the invented method is theability to sinter in air and at atmospheric pressure (i.e.,pressure-less sintered) to produce the fibrous monoliths, therebyobviating the need for hot pressing in an inert atmosphere to accomplishdensification. This leads to a reduction in cost of many FM parts by afactor of up to 100.

[0014] Still another object of the present invention is to provide aprocess for producing fibrous monoliths which are stable at a myriad ofoperating conditions. A feature of the process is the utilization ofdifferent particles sizes of the same compound for both the cell phaseand the cell boundary layer. An advantage of the invented process is theelimination of material compatibility problems, thereby also eliminatingdiffusion between phases so as to permit operation at highertemperatures. Stability in oxidizing, inert and reducing temperaturesalso is realized.

[0015] Yet another object of the present invention is to provide layeredmaterials comprised of multi-phased filaments. A feature of theinvention is that when the filaments are juxtaposed or “laid up” next toeach other prior to pressing, no voids exist between the filaments. Anadvantage of the invention is that eventual packing of these compositestrands results in very dense structures, sans any voids or deformationsbetween the filamentous components, so as to render the materials withhigh strength and toughness.

[0016] Briefly, the invention provides for a method for producingcomposite ceramics, the method comprising simultaneously forming aceramic core coaxial with a ceramic sleeve.

[0017] Also provided is a fibrous monolith comprising a plurality ofmulti-phase filaments arranged in a configuration to cause gracefulfailure of the monolith when the monolith is placed under mechanicalstress.

[0018] A fibrous monolith comprising a plurality of two-phase filamentscontaining ZrSiO₄ is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] These and other objects and advantages of the present inventionwill become readily apparent upon consideration of the followingdetailed description and attached drawing, wherein:

[0020]FIG. 1A is a schematic depiction of the invented co-extrusionprocess, in accordance with features of the present invention;

[0021]FIG. 1B is a view of FIG. 1A taken along line B-B;

[0022]FIG. 1C is a schematic depiction of a continuous hopperconfiguration, in accordance with features of the present invention;

[0023]FIG. 1D is a schematic diagram of the invented co-extrusionprocess and coating process, in accordance with features of the presentinvention;

[0024]FIG. 2 is a schematic depiction of a process for producingunidirectional fibrous monoliths from the invented duplex filaments, inaccordance with features of the present invention;

[0025]FIG. 3 is a stress v. displacement curve for typical monolithicceramics;

[0026]FIG. 4 is a stress v. displacement curve for a fibrous monolithproduced from duplex filaments produced via the invented method, inaccordance with features of the present invention;

[0027]FIG. 5 is a thermal expansion curve for the ZrSiO₄ constituents ina duplex filament, in accordance with features of the present invention;

[0028]FIG. 6 is a photomicrograph depicting stress faults through theinvented construct, in accordance with features of the presentinvention;

[0029]FIG. 7 is a schematic depiction of deformation occurring inpacking ovoid-shaped or round fibrous monoliths;

[0030]FIG. 8 is a perspective view of laid-up non-ovoid filaments, inaccordance with features of the present invention;

[0031]FIG. 9 is a cross section of a two phase filament willround-corner cell phase, in accordance with features of the presentinvention; and

[0032]FIG. 10 is a perspective of a graded fibrous monolith structure,in accordance with features of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0033] A low-cost, highly optimized method for producing fibrousmonoliths from duplex ceramic filaments is provided. Salient features ofthe invented method include the simultaneous formation and envelopmentof a ceramic core phase with a ceramic sleeve phase. This simultaneousformation is the result of the continuous extrusion of FM filaments,including solid freeform fabrication, so as to achieve net-shapefabrication of FMs.

[0034] Generally, the core phase is relatively dense compared to thesleeve phase, the latter commonly referred to as the cell boundaryphase.

[0035] The resulting duplex filaments represent a composite structurecontaining a strong continuous cellular phase surrounded by a secondphase that is designed to dissipate energy during failure. This secondor surrounding boundary phase has a non-brittle fracture characteristicto allow for gradual splitting and delamination.

[0036] Specifically, metal oxide-based fibrous monoliths (FMs)containing porous cell boundaries and dense cells were fabricated bytheir simultaneous extrusion and subsequent lay up of filaments. Theresulting construct was then sintered. Resulting bars of the materialexhibited significant energy dissipation during fracture and retainedsome load to large displacements. The FM bars exhibited clear evidenceof crack deflection and delamination.

[0037]FIGS. 1 and 2 provide a schematic depiction of the process. Theco-extrusion process, which produces the duplex filaments, is depictedas numeral 10 in FIG. 1. Cell material 12 from a screw extruder (riotshown) enters the die 14 along the latter's longitudinal axis β. Thecell material enters the die through a first injection port 16.

[0038] The screw extruder allows cell material to be continuously fedinto the first injection port 16. As such, plug size is no longer alimiting factor. The invented continuous hopper design makes it possibleto produce any length cell material desired. This configuration alsostreamlines FM fabrication procedures. For example, spoolable filamentscan be produced.

[0039] Although screw extrusion is the preferred approach to producingcontinuous multilayer filaments, ram extrusion also can be used.

[0040] A preferred method for continually feeding cell and/or boundaryphase material into the filament die 14 is depicted in FIG. 1C.Generally, a screw-driven machine, designated as numeral 11 is utilized,material is fed into an open bore 15, from a continuous feed hopper 17.An axial rotating screw 19 forces the introduced material through anexit orifice 31.

[0041] This continuous method allows for the substitution of cell and/orboundary materials in “mid-stream”, inasmuch as individual hoppers canbe utilized for different materials or for continuous production ofmultilayer filaments. In a ram extruder, multiple slugs can be utilizedwithout worrying about the joint or interface between plugs. Thismultiple hopper configuration thus facilitates the production of a cell-and/or boundary-phase comprised of a single constituent phase or ofdifferent materials along its linear axis, creating a linear variabilityof phase constituency. This linear variability imparts differentstrengths and durabilities of the phase(s) along its (their) length(s).

[0042] Alternatively, multiple hoppers facilitates the production offilaments of any length, inasmuch as the hoppers can be fedcontinuously.

[0043] The filament 31 emanating from the hopper/screw configuration isthen utilized either as a feed for cell phase material (at the firstingress point of the extruder, element 16 FIG. 1A), or as a feed for theboundary phase material at the second ingress point (element 18 of FIG.1A).

[0044] Simultaneous with the injection of the cell material, cellboundary material 18 from a second screw extruder (also not shown)enters the die 14 from a second injection port 20. A duplex filament 22is produced having a cell or core 21 and a boundary phase 23. Thisaspect of the production process can be performed at room temperature,inasmuch as the plastic materials representing the injectable core andsleeve matter contain no thermoplastic materials that require heating tosoften into a “workable” state. As such, the instant plastic materialsare workable at room temperature.

[0045] These filaments 22 can be stored for up to several days.

[0046] Filament Manipulation Detail

[0047] Once the filaments are formed, they can be pretreated in a myriadof ways, prior to being formed into bundles. For example, filaments 22emanating from the die 14 can be coated with a material to impart athree- or four- or more-layered structure to the filament. Coatings alsocan be employed to impart a desired texture to the outer surface of theresulting multi-phase filaments so as to increase friction and thereforestymie pull-out tendencies. Coatings also can be applied to impartadditional ceramic layers, thereby fortifying the final strengthcharacteristics of the resulting multi-phase filament.

[0048] An exemplary process for applying coating is illustrated in FIG.1D. Briefly, duplex or multilayer filament 22 extruded from the die 14is routed to a means for contacting the filament with a desired coating.For example, a coating chamber 33 is utilized after filament formationto apply a third phase 35 to the outside surface of the filament 22.Other coating means include dip coating, spray coating, painting, ordoctor-blade wiping.

[0049] The constituency of the coating will vary, depending on thecharacteristic desired by the coating. In the case of laying down anadditional ceramic phase, approximate coating constituent values are asfollows, plus or minus 30 percent of each value:

[0050] 25 weight percent ceramic powder;

[0051] 58 weight percent solvent;

[0052] 14 weight percent binder;

[0053] 2 weight percent plasticizer; and

[0054] 1 weight percent dispersant.

[0055] The solvents, binders, plasticizers and dispersants utilized arestandard for the ceramic process industry. Suitable constituents includexylene and/or butanol for solvents, thermosetting acrylic polymers forbinders, butyl benzyl phthalates for plasticizers and fish oil fordispersant. Exemplary constituents include 78/22 weight percentxylene/butanol as solvent, Rohm & Haas AT-51 binder, Monsanto S-160plasticizer and Menhaden fish oil or similar standard dispersant.

[0056] Filament Arrangement Detail

[0057] Ultimately, the filaments are formed into bundles 24. As depictedin FIG. 2, the filament bundle 24 is placed in an extrusion dye 26 andextruded to form unidirectional fibrous filaments 28. A punch 30 orsimilar device is utilized to force the bundle through the exit orifice32 of the extrusion dye. The masses for coextrusion (both core andsleeve) produce initial torque values of 4-12 newton meters. Thecoextrusion process requires a force of 5 to 50 MPa, most commonly 10-40MPa. The ram extrusion process requires a force of 10-50 MPa, mostcommonly about 15-28 MPa.

[0058] The resulting FM filaments are then subjected to a lay upprocesses. In one lay up scenario, FM filament is first formed intosheets by, for example, sectioning the filament into lengths andarranging them side by side in a plate or bar die. Instead ofside-by-side juxtaposition, the filaments can be serpentined along theplane in a die defining the sheet, or even patterned into a sheet in asolid free form fabrication. The lay up process is completed by stackingand shaping the sheets to form FM test specimens.

[0059] The inventors have devised a graded structure, depicted in FIG.10 as numeral 40, as a fibrous monolith configuration. In a preferredgraded embodiment, peripheral regions 42 of the monolith 40 containfilaments comprised of 60-90 volume percent cell phase. By contrast, theregion of the monolith extending inwardly 44 (i.e., medially directedaway from the periphery, contains filaments with at least approximately5 volume percent lower cell phase, compared to its boundary phase.Generally, lower cell phase volume percents in the inner regions imparthigher toughness to the body of the structure and higher cell phasevolume percents impart higher strength to the periphery region of thestructure, which is typically subjected to higher stress.

[0060] An exemplary volume percent ratio in such a graded structure is80/20 cell/boundary phase volume percent for periphery filaments and ≦75percent cell phase/≧25 percent boundary phase for filaments residing inthe inner portion of the monolith.

[0061] After laying up (which can be done by hand or with use of anautomated system) and pressing the filaments into desired monolithconfigurations, the resulting structures are then sintered attemperatures ranging from approximately 1000° C. to 1600 ° C. for a timesufficient to achieve the desired density of the core phase and theporosity of the surrounding sleeve. Alternatively, instead of beingsubjected to sintering, the resulting structures are hot-pressed atapproximately the same temperatures.

[0062] Four point flexural tests were conducted on the cell material,cell-boundary material and FMs. FIG. 3 depicts the stress-displacementcurves for the core material (solid line) and sleeve material (dottedline) before their combination to form a duplex filament. FIG. 4 depictsthe stress-displacement curve for the FM which represents a combinationof the core and sleeve material. Both the core material and the sleevematerial, when tested separately, exhibited fast fracture at maximumstresses of approximately 220 MPa and 430 MPa, respectively.

[0063]FIG. 4 shows FM bars failing at higher strain (compared to thematerials depicted in FIG. 3), as evidenced by the large area under itscurve after the maximum load has been attained, and also as depicted bythe jagged, downward curve between 0.1 mm displacement and 0.13 mmdisplacement. FIG. 4 shows three key features that provide the toughnesscharacteristic of the FM material. The first feature is non-linearityprior to reaching maximum load. This feature is depicted in region A ofFIG. 4 which shows a slight downward deviation of the curve or a slightweakening of the construct between 0.08 mm and 0.105 mm displacement.The bending illustrated in region “A” depicts the small scalefracturing, which is indicated by acoustical or thermal signature. Suchsignatures serve as a warning that the component stress should bedecreased to avoid failure.

[0064] The second feature, designated as “B”, is a series ofunload/loading steps during crack propagation. This second featurerepresents an intermediate zone of fracturing of the construct, withsplintering of the construct occurring more rapidly than that seen inthe first phase (i.e. region “A”) of failure.

[0065] The third feature of the invented FM is its retention of smallload-carrying capability to very large strains. As depicted in region Cof FIG. 4, some adherence remains in the construct, even after aprecipitous drop in strength, so that approximately 5 to 7.5 MPa ofyield is evident.

[0066] Scanning electron microscopy indicated that the unloading stepsobserved in the FM stress v. displacement curves were caused byenergy-dissipating events such as crack delamination, crack deflection,and some cell pull-out. As depicted in FIG. 6, the inventors have foundthat primary cracks follow tortuous paths through the FM, making several90 degree deviations through the cell-boundary phase 23 beforecontinuing through the FM. Point 25 in FIG. 6 represents the fractureorigin.

[0067] Cell and Boundary Configuration Detail

[0068] Strength of the resulting FMs is further enhanced if theindividual filaments 22 are configured to shapes to facilitate densepacking of the filaments. As such, in some instances, round filamentsare to be eschewed to prevent the formation of void space orinterstitial spaces (see “I” in FIG. 7) between laid up filaments. Whenthese filaments are subsequently compressed to make monoliths,particularly when cross-plies are made, a certain degree of collapsingor caving-in occurs to fill those interstitial spaces, thus formingcusps 40. This leads to distortion. Such distortion presents potentialpoints of breakage, particularly when the angle between adjacentfilament layers is 90 degrees.

[0069] To avoid the creation of spaces, rectangular or polygonalfilaments instead are produced and juxtaposed to each other, as seen inFIG. 8. As such, resulting cross-ply laminates contain fewer and muchsmaller distortions. Compared to the laminate depicted in FIG. 7, theamount of energy required to pull these fibers out of a FM of the typeillustrated in FIG. 8 is higher; therefore, graceful failure and highertoughness is achieved.

[0070] Alternatively, void space between juxtaposed round filaments areminimized if the round filaments are first flattened prior to firing(i.e., while the organics are still present in the filaments.)Flattening can be accomplished by pressing or rolling. Inasmuch as theflattening process causes the filaments to take on a rectangularconfiguration, void spaces are removed. The resultant sheets can then bereadily stacked.

[0071] The inventors also learned that cell phases which vary in crosssection compared to boundary phase filaments can result in addedstrength to resulting fibrous monoliths. For example, rounding the“corners” of cell phase cross sections minimizes the severity of stressconcentration points, thereby imparting higher strength to the cellphase during loading. A cross section of an exemplary composite fiber isdepicted in FIG. 9.

[0072] Generally, no limits on the radius of curvature are required.Preferably, however, the radius would fall into the followingapproximate range:

0.02a≦r≦0.2a

[0073] whereby “a” is the dimension of the side of a polygon that is thecross section, and “r” is the radius of curvature of the round cornersof the cell phase. Setting r to between {fraction (1/50)} and ⅕ of aside is suitable.

[0074] The inventors found that triangles, squares, trapezoids,rectangles and hexagons are particularly suitable cross-sectionconfigurations for the cell phase.

[0075] Particle Preparation Detail

[0076] A salient feature of the invented processes the production of FMswhich are produced in situ from oxide powders. The graceful failuredepicted by the invented FM is the result of tailoring particle sizes ofthe two constituents (i.e. the core or cell, and the sleeve or cellboundary phase). The inventors have found that the density and relativestrength between the core and sleeve phases is controlled by controllingparticle sizes. To create the necessary microstructure of the resultingfibrous monoliths, the inventors utilized milling and settlingtechniques to create well-controlled particle size distributions.Generally, particle sizes of the constituents comprising the corematerial are smaller than the particle sizes of the oxides comprisingthe sleeve material. Core particle sizes range from approximately 0.3microns (μm) to 3 μm. The sleeve's oxide powders contain particle sizesranging from 5 to 50 μm.

[0077] The powders of specific size become constituents in a firstplastic formulation for the core extrusion process and a second plasticformulation for the sleeve extrusion process. Generally, each of theprepared powders were mixed with organics and vibratory milled for atime sufficient to create the desired particle sizes and dispersion.More detailed formulation details are provided in Example 1 below forZrSiO₄ powders.

[0078] While the example contained herein deals with a ZrSiO₄ oxide, amyriad of oxides also are candidates for the plastic formulationprocess, including, but not limited to, Al₂O₃, mullite, yttrium aluminumgarnet, or combinations thereof. Generally, materials exhibiting goodhigh-temperature properties are suitable. Through doping and control ofpowder particle size, these materials are processed over a range ofdensities and strengths.

[0079] Reduction in the concentration of fine oxide powder in the cellboundaries further weaken the cell/boundary interface and promoteadditional delamination and deflection.

[0080] Residual Stress Detail

[0081] The inventors have found that residual stresses can be built intofibrous' monoliths, leading to a substrate which arrests propagatingcracks. Residual stresses are established when two materials are bondedwell to each other and cool together from a processing temperature. Ifthe materials have different coefficients of thermal expansion (CTEs),stresses are created.

[0082] Residual stresses arise from differences in thermal expansioncoefficients between the cell and various surrounding layers in thefibrous monolith. The sum of the residual stresses will be zero. Ifresidual stresses are engineered into strategic locations of a monolith,crack propagation can be arrested. A stress of approximately 1 GPa orgreater can arrest a crack. More detailed discussion of engineeringresidual stresses in multilayered structures appears in D. J. Green etal., Science 283, 1205-1297 (1999); and M. P. Rao et al. Science 286,102-104 (1999), and incorporated herein by reference. However, theapplication of residual stresses in fibrous monoliths are unknown.

[0083] Surprisingly and unexpectedly, the inventors found that residualstresses are established in FMs by controlling the concentration ofoxides in the cell and boundary layers, and that a sintered fibrousmonolith with these residual stresses can be prepared. These fibrousmonoliths remained intact; the stresses did no induce failure. Forexample, controlling the concentration of such materials as aluminumoxides, mullite, zirconium oxide, or yttria-alumina garnet will modifythe residual stresses. Generally, the materials are selected so thatthey (a) bond, (b) are compatible with each other, and (c) have CTEsthat are the right difference (CTE_(cell)−CTE_(−boundary)=ΔCTE) so thatfor a given ratio of volume of cell to volume of boundary material andfor a given sintering or processing temperature, the targeted residualstresses are established.

[0084] The inventors assume an average ΔT (processing temperature toroom temperature) of approximately 1000° C., with a range of betweenapproximately 600-1600° C. For any given ratio of cell to cell boundary(by volume fraction) the inventors have devised a ΔCTE range to providethe target stresses. Table 1 lists target coefficient of temperatureexpansion values for exemplary cell material/boundary material volumepercents. TABLE 1 Coefficient of Thermal Expansion for specificcell/boundary ratios Ratio in Volume % ΔCTE (° C.⁻¹) 90/10 0.2 × 10⁻⁶80/20 0.9 × 10⁻⁶ 70/30   2 × 10⁻⁶ 60/40 3.7 × 10⁻⁶

[0085] The values listed in Table 1 depend on the exact temperature andslightly on configuration. As such, the variability of the ΔCTEs listedmay be as much as 50 percent. Table 2 below provides residual stressdata for exemplary fibrous monoliths. TABLE 2 Residual Stress data forselected fibrous monoliths Resid. Resid. Stress Stress Core SheathVolume Core Sheath Core CTE Sheath CTE Ratio (MPa) (Mpa) Material (1/°C.) Material (1/° C.) (cell/sheath) Tensile Compress. ZTM* 8.75 MTA*7.55 70/30 1500 700 (50/50) 50/50 half-half half-half by volume byvolume ZTA* (10/90) 9.44 MTA 7.55 80/20 300 3100 90% alumina (50/50) byvolume Alumina 9.2 Mullite 5.9 80/20 200 3400 ZTA (10/90) 9.44 MTA 8.8780/20 120 1200 Mullite 5.9 MTA 7.55 80/20 540 1900 (50/50) 70/30 1700900

[0086] Cell- and Boundary-Phase Dimension Detail

[0087] Aside from manipulating the constituents of the two phases, orthe shape of the two phases, the invented device facilitates rapidmodification of the distances between the laminar phases and also thefinal extrusion shape of the composite fiber.

[0088] For example, the diameter of the core pin as seen in FIG. 1 isconfigured to provide the desired thickness of the boundary layer,defined as the thickness of the annular space between the pin and die14. This annular space determines the thickness of the boundary layersurrounding the cell layer and therefore the area fractions of thecore/sleeve phases. Typically, thicknesses of between approximately 0.01millimeters (mm) and 0.1 mm for the boundary layer are targeted.

[0089] The utilization of a plurality of core pins will facilitate theextrusion of three, four, or multi-layer filaments. In such an instance,the pins are successively arranged coaxial with each other, so as toextend along the longitudinal axis of a longer die than typically usedwhen just one pin is present. For example, in the generation of athree-layer filament, a first pin would generate the starting cellphase. A second pin, proximal to the final exit point of the filamentand therefore intermediate between the exit point and the first pin, isadapted to receive the just-produced two-phase filament to facilitateproduction of the three-layer filament. In essence, the second pin isarranged “down-stream” from the first pin.

[0090] In addition, and as also depicted in FIG. 1B, which is a view ofFIG. 1 taken along line B-B, an end cap 13 of the die is configured toeffect a desired cross section of the double-phase filament. FIG. 1Bdepicts a rectangular exit orifice of the die, therefore, resultingextruded double-phase filaments will be rectangular in cross section.

EXAMPLE 1

[0091] ZrSiO₄ powders were obtained from Alfa Aesar of Ward Hill, Mass.,and Remet of Utica, N.Y. The Alfa Aesar powder was utilized for thedense cell (i.e. core) and the Remet powder was used for the porous cellboundary. Each of 20, these powders was processed differently. Theas-received Alfa Aesar powder had an average particle size of 1 μm. Itwas ball-milled in isopropyl alcohol with ZrO₂ milling media for 72hours, then dried and screened through a 100-mesh (150 μm, or 0.0059inches) sieve. The resulting particle size was 0.7 μm.

[0092] Remet flour-grade powder was first processed to remove the finestparticles. Three Remet powders were classified by sedimentation once,twice or thrice, with the average particle sizes for each of powdersbeing 13.5 microns, 22 microns and 29 microns, respectively. Theprincipal difference between the three powders is the fraction of finesthat remain after settling. Approximately 100 grams of the Remet ZrSiO₄powder, having an initial particle size of 7 μm, was placed in a 1000milliliter beaker with 800 milliliters of deionized water and 20 dropsof a deflocculent (i.e., a dispersing agent.) Darvan C, available fromR. T. Vanderbilt Company, Inc., or Norwalk, Conn. is a suitabledeflocculent. The solution was mixed for approximately one minute andthen allowed to stand for a time sufficient for the two phases todevelop, which in this instance was approximately 3 minutes. Theremaining solution was decanted and the settled material was retained.The settled powder had an average particle size of 13.5 microns.

[0093] After the ZrSiO₄ particles are sieved, their plastic processingbegins.

[0094] In a first step of plastic processing, the necessary compositionmust be batched and then subjected to a vibratory or ball mill for atime sufficient to create an homogenous mixture. Table 3 shows thecomponents and concentrations that were used in exemplary plastic mixes.Generally, the plastic mix constituents include the ceramic powders, abinder, a solvent, a plasticizer and a deflocculant. Generally, thebinders serve to hold the particles together. A myriad of binders arecommercially available and suitable for incorporation in the instantformulation. Suitable binders are medium to long-chain polymers with endfunctional groups. The ends attach to the powders and in so doing,provide a measure of strength to the mixture. AT-51 Binder from Rohm &Haas, Philadelphia, Pa. is one such suitable binder.

[0095] Plasticizers are utilized in the mix so as to modify thestructure of the binder to make the later more flexible. An exemplarybinder is butyl benzyl phthalte, marketed as S-160 Plasticizer fromMonsanto, Fayetteville, N.C. S-160 is a short-chained polymer.

[0096] Deflocculents prevent agglomeration of the powders by minimizingelectrostatic attractions between the powders. Fish oil is a standarddeflocculant.

[0097] Solvent is utilized to provide homogeneity to the organic mix. Itis the vehicle in which the organic constituents dissolve. As such, amyriad of solvents are suitable for the invented process. Xylene/butanolis an exemplary solvent.

[0098] Carbon powder is a constituent specific for the cell boundaryslip. It serves to minimize the shrinkage differential between the cellphase and the boundary by leaving voids after burn off of the sleeve.The presence of the voids is short-lived inasmuch as the sleeve layerconstituents cave in to fill the voids. The carbon also is used todifferentiate the sleeve phase from the cell phase inasmuch as theceramic powder is white and the carbon is black. TABLE 3 Composition ofplastic mixes used for extruding ZrSiO₄ filaments Alfa Aesar RemetConstituent ZrSiO₄ (g) ZrSiO₄ (g) 78 wt % xylene/22 wt % butanol 20 15Monsanto S-160 Plasticizer 10 10 Dilute fish oil in xylene/butanol 10 10ZrSiO₄ powder 200 200 Rohm & Hass AT-51 Binder 21 21 Carbon Powder 00.25-5

[0099] After the mixes were milled overnight, they were de-aired, tapecast to a thickness of approximately 0.5 mm, substantially dried andstripped. The tapes were allowed to sit overnight and were then mixed ina Brabender high-shear mixer. Mixing was used to adjust viscosity to thenecessary levels for co-extrusion. Inasmuch as not all solvent isremoved during the drying process, the “dried” mixes retained someflowability. Essentially, the drying increases viscosity of the mix.

[0100] In the co-extrusion process, depicted in FIG. 1, two Brabenderscrew extruders were attached to the co-extrusion die 14. The Alfa Aesarplastic mix was introduced into the first entry port 16 (i.e., a firstextruder hopper) of the die and the Remet plastic mix was fed into asecond entry port 18 (i.e. second extruder hopper) of the die. The twoseparate plastic mixes were simultaneously forced into the extrusion dieto produce an initial filament 22.

[0101] The filaments were cut into 10 centimeter sections and bundled,as depicted as element 24 in FIG. 3. The bundle was then placed in anextrusion die and extruded to produce a fibrous monolith filament 28.

[0102] The fibrous monolith was cut into 50 mm sections, stacked in abar die, and pressed in a bar die at a pressure of approximately 100MPa. The resultant bars were then heat treated. Binder burnout wasaccomplished in flowing O₂ or N₂. Binder can be burned out in anyoxidizing or inert atmosphere. Each bar was heated to 140° C. at 50°C./hour and held for 0.1 hours. After the hold, each bar was heated to500° C. at 5° C./hour, held for 3 to 6 hours, and then cooled to roomtemperature at 50° C./hour. The bars were sintered in air at 1550° C.for 3 hours to complete the processing of the ZrSiO₄ FMs.

[0103] Thermal expansion curves for 95 percent dense ZrSiO₄ (i.e., thecore material) and 70 percent dense ZrSiO₄ (i.e., the sleeve material)are depicted in FIG. 5. These expansions are close enough such thatthermal-expansion mismatch will not cause significant cracking of theFM. The FM bars exhibited a 23 volume percent shrinkage between initialpressing and the fired state.

[0104] The FM bars were approximately 70 volume percent cell (i.e.,core) and 30 volume percent cell boundary (i.e., sleeve). This is lowerthan the ratio of 85 vol. percent cell and 15 vol. percent cell boundarygenerally observed for Si₃N₄/BN FMs. The average cell size (i.e. corediameter) of the ZrSiO₄/ZrSiO₄ FMs was approximately 150 microns.

[0105] The samples were loaded in an Instron Model 4505 tester,available from Instron Corporation, Canton, Mass.

[0106] Scanning electron microscopy revealed a significant difference inshrinkage between the cell (i.e. core) and cell boundary (i.e., sleeve),with the cell shrinking more than the sleeve. This shrinkage willfacilitate slippage at the interface between the two phases during afracture. This, combined with a pullout of individual cores, aid indissipating energy.

[0107] The resulting FM product from the invented process confersseveral benefits, including being stable under oxidizing, inert andreducing atmospheres. Inasmuch as it consists of one constituent, it isinherently stable over time.

[0108] Aside from the production of two-phase FMs, the inventors alsohave developed a three-phase FM which contains a cell, an interphase,and a matrix. The cell and interphase are made by co-extrusion. Thematrix is the effect of a bundling of various filaments. The matrix alsocan be produced via various infiltration techniques.

[0109] A feature of the three-phase FM is that the weakest phase in thestructure is not continuous. In a construct where non-continuous phasesare utilized, any crack is isolated to a cell or group of cells. In suchnon-continuous phase structures, crack delamination, especially fromout-of-plane loading, occurs discreetly from one cell to the next witheach interphase acting as a crack trap. This differs from a continuousphase system wherein energy (manifested as a crack or delamination)traverses the entire structure. Thus, the advantage of thisnon-continuous construct is that greater strength and fracture toughnessresult.

[0110] While the invention has been described with reference to detailsof the illustrated embodiments, these details are not intended to limitthe scope of the invention as defined in the appended claims. Forexample, while only two extruder hoppers are depicted in the instantfilament-production process, additional hoppers could be added“downstream” from the point where the first sleeve is first laid down.As such, these third or fourth layers are applied distal to the firstsleeve hopper, relative to the injection port utilized by the corematerial. This will facilitate the production of multiplex filamentssuch as triplex and quadraplex filaments. A head-on view of such aresulting filament would resemble a tree-ring configuration wherein thelayers are concentrically aligned and co-axial to each other.

[0111] Also, while neat formulations of ZrSiO₄ were utilized in theexamples herein, the utilization of this oxide, as well as other oxides,containing dopant is within the scope of the instant teaching. Theaddition of dopants, such as yttrium oxide, boron oxide or boric acid,alkali metal hydroxides (such as sodium hydroxide), magnesium oxide,titanium dioxide and lead oxide further provide a wider range ofdensities for the core and cell boundary layers. Such dopants would bepresent in weight percents of approximately less than 2 weight percent.

The embodiment of the invention in which an exclusive property orprivilege is claimed is defined as follows:
 1. A method for producingcomposite ceramics, the method comprising: simultaneously forming aceramic core coaxial with a ceramic sleeve.
 2. The method as recited inclaim 1 wherein the filament and the core are co-extruded in acontinuous process.
 3. The method as recited in claim 1 wherein the coreand the sleeve are comprised of identical compounds.
 4. The method asrecited in claim 1 wherein the core and sleeve are comprised ofcompounds selected from the group consisting of ZrSiO₄, Al₂O₃, mullite,yttrium aluminum garnet, or combinations thereof.
 5. The method asrecited in claim 2 further comprising: a) forming material, comprisingconstituents of the core, into a first plastic mass; b) forming materialcomprising constituents of the sleeve into a second plastic mass; c)forcing the first plastic mass into a first entry port of a co-extrusiondye while simultaneously forcing the second plastic mass into a secondentry port of the co-extrusion dye so as to produce a duplex filamentwherein the core is coaxial to and surrounded by the sleeve; d)repeating steps a-c until a desired number of filaments are produced; e)subjecting the produced filaments to a ram extrusion process to producea fibrous monolith; f) assembling the fibrous monoliths into apredetermined shape; and g) sintering the assembled fibrous monoliths.6. The method as recited in claim 5 wherein the produced filaments arearranged parallel with each other prior to the ram extrusion process soas to produce a unidirectional fibrous monolith.
 7. The method asrecited in claim 5 wherein the produced filaments are arranged atvarious angles to each other to form a multidirectional fibrousmonolith.
 8. The method as recited in claim 5 wherein the fibrousmonoliths are arranged in a configuration so that fibrous monoliths in aperipheral region of the configuration is comprised of 60-90 volumepercent of the first plastic mass and fibrous monoliths in a regionmedially directed from the peripheral region have at least 5 percentlower volume percent of the first plastic mass.
 9. The method as recitedin claim 5 wherein the first plastic material is formed by a) sizingcompounds comprising the core to between 0.3 microns and 3 microns indiameter; b) mixing the sized compounds with plasticizer, solvent andbinder so as to form a slurry; and c) homogenizing the slurry.
 10. Themethod as recited in claim 5 wherein the second plastic material isformed by a) sizing compounds comprising the sleeve to between 5 and 50microns in diameter; b) mixing the sized compounds with plasticizer,solvent, binder and carbon so as to form a slurry; and c) homogenizingthe slurry.
 11. The method as recited in claim 5 wherein theconstituents of the core comprise particles having diameters of between0.3 microns and 3 microns.
 12. The method as recited in claim 5 whereinthe constituents of the sleeve comprise particles having diameters ofgreater than 5 microns and less than 30 microns.
 13. The method asrecited in claim 1 wherein the core comprises particles and the sleevecomprises particles and wherein the particles of the core have a smallerdiameter than the particles of the sleeve.
 14. The method as recited inclaim 5 wherein the filaments have a circular cross section.
 15. Themethod as recited in claim 5 wherein the filaments have a rectangularcross section.
 16. The method as recited in claim 5 wherein the crosssection of the core contains round corners.
 17. The method as recited inclaim 5 wherein the cross section of the sleeve contains angularcorners.
 18. The method as recited in claim 6 wherein the filaments havea circular cross section and the filaments are flattened after beingarranged, to a configuration sufficient to cause the cross section toapproximate a rectangle.
 19. The method as recited in claim 5 wherein athird plastic mass is applied to the sleeve at a first point distal fromthe second entry point so as to produce a second sleeve concentric withthe core.
 20. The method as recited in claim 19 wherein a fourth plasticmass is applied to the second sleeve at a second point distal from thefirst point.
 21. The method as recited in claim 5 wherein the corecontains tensile residual stress and the sleeve has a compressive stressthat is sufficiently large to arrest crack propagation when thecomposite ceramic is put under load.
 22. A fibrous monolith comprising aplurality of multi-phase filaments arranged in a configuration to causegraceful failure of the monolith when the monolith is placed undermechanical stress.
 23. The monolith as recited in claim 22 wherein thefilaments each have a first phase and a second phase, whereby thesecond-phase is peripherally arranged about the first phase and coaxialto the first phase.
 24. The monolith as recited in claim 23 wherein thevolume percent of second phase in filaments comprising an inner regionof the monolith is higher than the volume percent of second phase infilaments comprising a peripheral region of the monolith.
 25. Themonolith as recited in claim 23 wherein the first phase has a crosssection containing rounded corners.
 26. The monolith as recited in claim23 wherein the second phase has a cross section containing angularcorners.
 27. The monolith as recited in claim 23 wherein the first phasehas a cross section containing rounded corners and the second phase hasa cross section containing angular corners.
 28. The monolith as recitedin claim 22 wherein no space exists between the arranged filaments. 29.The monolith as recited in claim 22 wherein the filaments have crosssections resembling triangles or squares or rectangles or trapezoids orhexagons or combinations thereof.
 30. The monolith as recited in claim23 wherein the first phase and the second phase contain oxides selectedfrom the group consisting of ZrSiO₄, Al₂O₃, mullite, yttrium aluminumgarnet, or combinations thereof.
 31. A fibrous monolith comprising aplurality of two-phase filaments containing ZrSiO₄.
 32. The monolith asrecited in claim 31 wherein in each filament, the two phases areconcentrically arranged relative to each other extend along thelongitudinal axis of the filament.
 33. The monolith as recited in claim31 wherein the filaments have cross-sections resembling triangles orsquares or rectangles or trapezoids or hexagons or combinations thereofof the filaments.